cmake-cxxmodules(7) on Linux
The text f256f6eedb37, as Debian 13 (Trixie) shipped it.
CMake C++ Modules Support Reference Added in version 3.28. C++ 20 introduced the concept of "modules" to the language. The design requires build systems to order compilations among each other to satisfy import statements reliably. CMake's implementation asks the compiler to scan source files for module dependencies during the build, collates scanning results to infer ordering constraints, and tells the build tool how to dynamically update the build graph.